Ionization manometer



Jan. 20, 1959 H. MOESTA 2,370,358

IONIZATION MANOMETER Filed D60. 26, 1957 Fi 1 10 l g i l 5O 26 N 27 -\gg Fig. 2

In re r1 ar Hasso Maesfa United States Patent 'IONIZATION MANOMETER Hasso Moesta, Koln-Weidenpesch, Germany Application December 26, 1957, Serial No. 705,406 Claims priority, application Germany December 31, 1956 5 Claims. (Cl. 313-7) The invention relates to vacuum-measuring instruments, to so-called ionization manometers.

In the known ionization manometers, the measured magnitude is the number of positive ions formed in a constant ionization chamber at a constant intensity of excitation, so that the number of positive ions is dependent only on the gas pressure and on the type of gas. Such a device comprises an electrons emitting cathode, a so-called grid, and a collector electrode or target for the positive ions formed. The electrons leaving the cathode are first accelerated towards the grid by a potential difference and cause a grid current which extends into the saturation range known in electronic valves. Thus in contrast to the usual operation of electronic valves, the grid is kept at a positive potential in relation to the cathode, and the applied voltage should be made so high that the saturation current flows towards the grid. Nevertheless, a great number of electrons accelerated by the grid potential passes through the widemesh grid into the space between the grid and the collector electrode and collides there, depending on the prevailing gas pressure, on a larger or smaler number of air molecules, which partially become ionized. The resulting positive ions are now attracted by the collector electrode which has the most negative potential in relation to the cathode, and produce a current which is strictly proportional to the value ofthe absolute pressure prevailing in the ionization chamber. In this manner, reliable measurements are obtained down to very low pressures (generally to 10- mm. Hg, but even down to 10 mm. Hg).

A practical construction of this manometer is represented by the measuring tubes of Bayard and Alpert, the construction of which may be said to be inverse in comparison with the usual arrangements. Known ionization manometers have generally a wire cathode, a cylindrical grid surrounding this cathode, and a cylindrical collector electrode surrounding the grid. In the Bayard and Alpert tubes, the collector electrode is centrally disposed, and the electrons emitting cathode is in the form of two V-shaped filaments situated outside a cylindrical grid which, as in electron valves, is constructed in the form of a helically wound wire fixed to two relatively thick supporting wires along two generatrices of the Wound cylinder.

The quality of such Bayard and Alpert measuring tubes depends largely on the electron-optical geometry of their construction and even slight variations in the relative position of the component parts may cause considerable variations in sensitivity. The construction accordingly demands extremely careful adjustment and is therefore rather difiicult to manufacture. Moreover, in general, in ionization manometers for low pressures, the component parts of the measuring system must be capable of being thoroughly heated so that no adhering gas residues are released during the measurement thus falsifying the result. This is likewise scarcely possible with the Bayard and Alpert tubes because the grid, which has to be wound from comparatively thick wire in order to have sufficient stiffness, requires very high current strengths for the degasification. To effect the required heating by electron bombardment would involve a heavy load on the cathode and therefore can hardly be carried out. In addition, there .is always the danger of distortion of the component parts as a result of the action of heat, and this would cause the faults already described. Such systems are therefore diflicult to heat for their degasification.

An increase in the sensitivity and equal quality of produced instrument can be. obtained according to a likewise known proposal by Nottingham, by keeping the disturbing influences of charges which appear on the glass walls of the tube, away from the measuring system by means of a screen electrode which is constructed in the manner of a Faraday screen. This screen electrode is of cylindrical form, and like the grid of the Bayard and Alpert tubes, consists of a helically wound wire which is fixed, along two generatrices of the Winding cylinder, to two relatively thick supporting wires.-

The possibility of thorough degasification by heating is thus further restricted by such a screen electrode.

One object of the invention is to devise an ionization manometer which, in comparison with the known devices, is less sensitive to variations in the relative position of its electrodes such as may occur as a result of minor inaccuracies in the assembly of the individual electrodes.

A further object of the invention is to provide an ionization manometer which is easy to manufacture and can be satisfactorily degassed by heating.

A still further object of the invention is to provide an ionization manometer which, in comparison with the known devices, is less sensitive to deviation from the prescribed dimensions of the individual electrodes.

The ionization manometer according to the invention comprises a rectilinear wire as a collector electrode, a cylindrical grid which is arranged concentrically to the collector electrode and which is built up from a plurality of wires which run, between two rings, along the generatrices of the grid cylinder, further, a circular wire cathode outside the grid cylinder and concentric thereto, and finally a screening electrode outside the cathode and concentric thereto.

. A plane determined by the circular cathode should preferably lie substantially at the same distance from the two rings of the grid cylinder.

Furthermore, the circular screen winding should preferably lie in the same plane as the cathode.

In another embodiment, the screening electrode consists of two circular wires, the planes of which are equidistant from the plane of the cathode wire.

The wires of the cylindrical grid may be constructed in the form of a tungsten spiral in the manner of ordinary bulb filaments.

Figure 1 of the drawing shows, in section, a preferred embodiment of the invention including the associated circuit connections, and Figure 2 shows, in section, another embodiment of the electrode system according to the invention.

The glassbulb 10, shown in Fig. 1, is adapted to be connected by means of a ground joint 11, to the vacuum vessel the vacuum of which is to be measured. The base 12 of the glass bulb 10 is connected to the bulb by fusion and contains a series of lead-in wires 13 to 18. In addiinto the glass tube 19 and is sealed in an airtight manner at the top of the glass tube 19. A grid electrode is arranged concentrically to the collector electrode 21 and consists of an upper and lower circular wire ring 23 and 24 between which run a series of wires along generatrices of the generally cylindrical grid. Altogether, the grid cylinder may, for example, comprise ten such wires, only six of which are illustrated in Figure l, and are designated 25 to 39. The upper and lower ends of these wires are secured to the annular rings 23 and 2-4 at equal distances by spot welding. According to Figure l, the wires 25 to 30 consist of tungsten spirals in the manner of ordinary bulb filaments. The whole grid electrode is secured to the lead-in wires 15 and 16 of which the lead-in Wire 15 leads to the upper ring 23 and the lead-in wire 16 leads to the lower ring 24. Coucentrically to this grid electrode lies the cathode 31, which consists of a circular filament, which is secured at two opposite points to the leadin wires 14 and 3'7, and held by them in a concentric position with respect to the grid electrode and the collector electrode. Also situated in the same plane as the circular cathode is a screen electrode 32 which is likewise in the form of a circular ring and is concentric to the cathode 31. The screen electrode 32 is secured to and held by the two lead-in wires 13 and 18 at diametrically opposite points.

Three transformers 33, 34 and 35, are connected in the conductors which lead to the various electrodes of the manometer. These transformers serve to heat the individual electrodes of the manometer strongly and hence to expel gas from them thoroughly before the beginning of the measurement. The secondary winding 36 of the transformer 33 is connected to the collector electrode 21 through the lead-in wire 29 and the wire 22 inside the glass tube 19. The secondary winding 37 of the transformer 34 leads via the two lead-in wires 13 and 18 to the two attachment points of the screen electrode 32, so that the secondary current from the transformer flows in parallel through the two halves of the screen electrode and brings it to a high temperature sufficient for the expulsion of gas. The heating current for expelling gas from the grid is supplied by the secondary winding 38 of the transformer 35 and flows through the two leadin wires 15 and 16. It flows through all the spiral grid wires 25 to 30 which are connected in parallel to one another in respect to the heating current.

The transformers do not disturb the vacuum measurement. The A. C. voltage is merely disconnected from their primary winds.

During the measuring operation, the hot cathode 31, is heated from a voltage source 40, for example of v., and the heating current fiows through the supporting wires 14 and and then in parallel through the two halves of the hot cathode. The grid electrode is held at a positive potential in relation to the hot cathode by means of a source of D. C. voltage 41, for example of 200 v., and a large proportion of the electrons emitted by the hot cathode passes between the wires to of the grid LII electrode and forms ions in the space between the grid electrode and the collector electrode. A negative potential in relation to the cathode is applied to the collector electrode 21 by means of a source of D. C. voltage 42 for example of v., so that the positive ions formed by the electrons which flow through the grid reach the collector electrode 21. This stream of ions is measured in an instrument 43 and, with a constant voltage of the batteries #16 and 41, that is to say with a constant temperature of the hot cathode 31 and constant acceleration as a result of the potential of the grid electrode, depends only on the gas pressure and the type of gas as in all ionization manometers.

The embodiment illustrated in Figure 2, only the electrode system of which is shown, differs from that in Figure l in only two respects. Firstly, instead of the tungsten spirals 25 to 30, there are used thin straight wires 25a to 30a, and secondly, instead of the circular screen electrode 32 in Figure 1, there is provided a screen electrode consisting of two circular wires 32a and 32b. The two circular wires 32a and 32b are situated at substantially the same distances above and below the plane of the circular hot cathode 31. The two rings 32a and 3217 are connected to the lead-in wires 13 and 18 through the supporting wires 43 and 44 in the same man ner as the circular screen electrode 32 shown in Figure l. The collector electrode 21 and the rings 23 and 24 of the grid electrode are likewise connected as illustrated in Figure 1.

All the electrodes of the measuring system in Figures 1 and 2 consist of a heat-resistant material such as tungsten, molybdenum or tantalum.

What I claim is:

1. An ionization manometer comprising a rectilinear wire as a collector electrode, a cylindrical grid which is arranged concentrically to the collector electrode and which is built up from a plurality of wires which extend between two rings along generatrices of the grid cylinder, a circular wire cathode outside the grid cylinder and concentric thereto, and a screen electrode outside the cathode and concentric thereto.

2. A manometer as claimed in claim 1, wherein the plane of the circular cathode is equidistant from the planes of the two rings of the grid.

3. A manometer as claimed in claim 1, wherein the screen electrode consists of a circular wire and is arranged in the plane of the cathode wire.

4. A manometer as claimed in claim 1, wherein the grid wires are constructed in the form of tungsten spirals in the manner of ordinary bulb filaments.

5. A manometer as claimed in claim 1, wherein the screen electrode consists of two annular wires, the planes of which are equidistant from the plane of the cathode wire.

References Cited in the file of this patent Yarwood: Ultra-high Vacuua, Journal of Sci. Insts., vol. 34, August 1957, pp. 298-299. 

